Fresnel focussed imaging system

ABSTRACT

An imaging system, particularly useful for acoustic medical diagnosis of a human subject, utilizes an array of radiating elements or sonic transducers located side-by-side and positioned along the subject. Signals received by the transducer are applied to a pair of pattern generation circuits which weight the individual signals by factors of +1, -1 or 0. Graphs of the weighting factors as a function of transducer location have the likeness of cosinusoidal and sinusoidal Fresnel patterns, these patterns being produced by the two circuits. Upon reception of signals, the weighted signals of each pattern are summed together, multiplied by cosinusoidal and sinusoidal reference signals and then summed together to provide a radiation pattern which converges from the array to a focal point in front of the array while eliminating a diverging pattern from a virtual focus behind the array. For transmission, sinusoidal and cosinusoidal signals are weighted by the Fresnel factors, summed together and applied to the transducers.

RELATED APPLICATION

This application is a continuation in part of an application entitled"Fresnel Focussed Imaging System" by Roger H. Tancrell having Ser. No.866,325 and filed Jan. 3, 1978, now U.S. Pat. No. 4,145,931.

BACKGROUND OF THE INVENTION

This invention relates to an imaging system employing an array ofradiation detectors and, more particularly, the use of a Fresnel patternimpressed upon signals received by the detectors to provide for imagingas in the case of the sonic imaging of a human subject.

Fresnel masking has been utilized in both electromagnetic and sonicimaging systems. With respect to electromagnetic imaging systems, aFresnel pattern is disclosed in the U.S. Pat. No. 3,263,079 which issuedto L. N. Mertz and N. O. Young on July 26, 1966 wherein the pattern isutilized for forming the image of stars in the sky. The use of a Fresnelpattern in nuclear medicine for forming an image of a radioactive sourceis disclosed in the U.S. Pat. No. 3,936,639 which issued in the name ofH. H. Barrett on Feb. 3, 1976. The use of a Fresnel pattern impressedupon the signals of sonic radiation detectors is disclosed in the U.S.Pat. No. 3,911,730 which issued in the name of L. Niklas on Oct. 14,1975 wherein the energization of groups of radiation detectors, ortransducers, is employed following the arrangement of a Fresnel patternin at least one dimension. The use of an ultrasonic imaging scanner forimaging organs of the human body is disclosed in the U.S. Pat. No.3,805,596 which issued in the name of C. N. Klahr on Apr. 23, 1974.

The use of the Fresnel pattern for sonic imaging systems is advantageousin that the Fresnel pattern provides for the focussing of the sonicradiation in the manner of a lens. A problem arises in that with systemsof the prior art, the Fresnel pattern, whether it be utilized with a onedimensional line array or in a two dimensional array, produces theeffect of both a converging pattern of radiation which converges towarda focal point in the subject in front of the array as well as adiverging radiation pattern which emanates from a virtual focus locatedbehind the array. The energy content of signals produced by thetransducers in response to incident sonic energy from the divergingradiation pattern approximately equals that of the energy content ofsignals associated with the desired converging pattern. As a result,there is substantial unwanted noise which degrades an image of thesubject obtained with the converging radiation pattern.

SUMMARY OF THE INVENTION

The aforementioned problem is overcome and other advantages are providedby an imaging system in conjunction with radiating elements such assonic transducers wherein, in accordance with the invention, a pair ofFresnel patterns is impressed upon signals of the transducers, oneFresnel pattern being a cosinusoidal Fresnel pattern while the second isa sinusoidal Fresnel pattern, for combining signals of the two patternsto produce a resultant radiation pattern of the array wherein theaforementioned undesirable diverging pattern is absent. Thereby, uponimaging a subject, such as a human being, various sites within thesubject are viewed by the converging radiation pattern to produce asharp image of each site without the interference associated with noisefrom the diverging radiation pattern. The imaging system of theinvention is equally applicable to an array of detectors ofelectromagnetic radiation as well as to an array of detectors of sonicradiation. However, for convenience in describing the invention,reference will be made to sonic transducers, it being understood thatthe description is equally applicable to the case of electromagneticradiation.

Each of the aforementioned Fresnel patterns is impressed upon signalsproduced by the transducers in response to sound waves incidentthereupon, or applied to the transducers during transmission, by a setof multipliers which are coupled to individual ones of the transducers.Each multiplier multiplies the polarity of a transducer signal by afactor of +1, -1 or 0. In a preferred embodiment of the invention, eachof the multipliers comprises an inverting amplifier with a selectorswitch which selects either the positive or negative output signals ofthe amplifier or provides for the grounding of the signal to provide thevalue of 0. The multiplication factor for each transducer signal isselected in accordance with the location of the respective transducerswithin the array so that a graph of the multiplication factors, as afunction of transducer location, has the appearance of a square waveapproximation to a Fresnel pattern. A pair of the multipliers is coupledto each of the transducers so that the aforementioned pair of Fresnelpatterns may be generated simultaneously.

For receiving signals from the subject, the sum of the products of themultipliers for the cosinusoidal Fresnel patterns are summed togetherand multiplied by a cosinusoidal reference signal. Similarly, theproducts of the multipliers for the sinusoidal Fresnel pattern aresummed together and multiplied by a sinusoidal reference signal. Theamplitudes of the products resulting from the multiplications with thetwo reference signals are then equalized and summed together. Theresultant sum is then passed through a band pass filter to removeharmonics of the multiplication operation and then passed to a displaywhereby the various sites within the subject may be seen. A controllerof the multiplying factors comprises a memory which is sequentiallyaddressed in accordance with the range or depth within the subject ofthe respective sites for altering the Fresnel pattern for focussing atthe respective sites whereby each of the sites is brought into sharpfocus. At the conclusion of the displaying of sites along a normal to agroup of transducers utilized in the Fresnel pattern, the multiplyingfactors are selected so as to shift the Fresnel pattern sideways alongthe array so as to focus on a contiguous portion of the subject.Continuous side-stepping of the Fresnel pattern permits the viewing of aswath or rectangular slice of the subject.

To transmit signals from the transducers, sinusoidal and cosinusoidalcomponents of the signals are each weighted in accordance with Fresnelpatterns and combined to produce the signals for transmission by each ofthe transducers.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned aspects and other features of the invention areexplained in the following description taken in connection with theaccompanying drawings wherein:

FIG. 1 shows a transducer array of the invention positioned in contactwith a subject, such as a portion of a patient in a hospital, thetransducer array being coupled to a transmitter and receiver for thetransmission of acoustic signals into the subject and the reception ofechoes therefrom;

FIG. 2 shows a plan view of the transducer array of FIG. 1 taken alongthe lines 2--2 of FIG. 1;

FIG. 3 shows a side view of the transducer array of FIG. 1 incombination with a graph displaying the temporal location of a group oftransducers of the array actively participating in the formation ofimages of sites within the subject of FIG. 1, the graph portraying theside-stepping of the active region for scanning a rectangular swath ofthe subject;

FIG. 4 shows a set of four graphs in registration with each other, thefirst graph showing a Fresnel function with the horizontal distancealong the transducer array of FIG. 1 serving as a parameter thereof, thesecond graph showing a square wave approximation to the Fresnelfunction, the third graph showing the multiplication of the signals ofindividual ones of the transducers of the active region of the arraywherein the polarity of the signals subsequent to the multiplicationfollows the pattern of the square wave approximation of the secondgraph, and the fourth graph shows the resultant converging radiationpattern, as well as the diverging radiation pattern which is obtainedwhen only one Fresnel function is employed in the signal processing;

FIG. 5 shows two graphs of the intensity of echo strength from a sitewithin the subject as a function of angle about a normal to the face ofthe transducer array, the normals and the angles being seen in thefourth graph of FIG. 4, the second graph showing the reduction in noiseresulting from the cancellation of signals of the diverging beam inaccordance with the invention;

FIG. 6 shows a block diagram disclosing the electrical connectionsbetween the transducer array and a transmitter and a receiver of FIG. 1;

FIG. 7 is a block diagram of the receiver of FIGS. 1 and 6 disclosingthe multiplication of the transducer signals to provide the pair ofFresnel patterns;

FIG. 8 is a block diagram of a controller of the multiplying factors ofFIG. 7, the controller including a memory storing the multiplicationfactors for the group of active transducers and a switching matrix forredirecting the factors as the group of active transducers is displacedsideways along the array for scanning the subject;

FIG. 9 is a block diagram of a signal splitter in the transmitter ofFIG. 6 for transmitting a beam focussed at infinity;

FIG. 10 is a block diagram of an alternative embodiment of the signalsplitter of FIG. 9 providing a Fresnel weighting for focussing thesignal within the subject;

FIG. 11 shows a block diagram of an alternative embodiment of atransmitter of FIG. 6 for providing sinusoidal and cosinusoidalcomponents of the signal;

FIG. 12 is an alternative embodiment of a signal splitter for weightingthe sinusoidal and cosinusoidal signal components for the transmitter ofFIG. 11;

FIG. 13 is yet another embodiment of the signal splitter for thetransmitter of FIG. 11 showing weighting by attenuators;

FIG. 14 is an alternative embodiment of the controller of FIG. 8 forsteering a radiation pattern of the array of FIG. 1, the controller ofFIG. 14 being used in the signal splitter of FIG. 13; and

FIG. 15 is a diagram of a Fresnel construction for a focal pointsituated away from a normal to the radiating aperture of the array.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is seen an imaging system 20 which, inaccordance with the invention, comprises a transducer array 22positioned in contact with a subject 24, and an amplifier system 26. Theamplifier system 26 and the transducer array 22 are incorporated withina common module 28. The system 20 further comprises a transmitter 30, areceiver 32 and a flexible cable 34 joining the transmitter 30 and thereceiver 32 with the module 28. A coordinate system 36 having X and Yaxes is situated at a corner of the module 28 at the surface of thesubject 24 for locating sites within the subject 24, the X axismeasuring horizontal positions along the interface between the module 28and the subject 24 while the Y axis measures depth into the subject 24from the face of the transducer array 22. Also shown are sound rays 38emanating from sites within the subject 24 to illustrate propagation ofechoes from the sites to the transducers of the active portion of thearray 22.

As will be seen subsequently, the transmitter 30 transmits a pulseelectrical signal via the cable 34 and the amplifier system 26 to thetransducer array 22, individual tranducers 40 of the array 22 beingselectively energized by the pulse signal for transmitting a sonic pulsesignal toward the sites in the subject 24. Echo signals propagating fromthe sites back to the array 22 are coupled via the amplifier system 26to the receiver 32 which performs a Fresnel multiplication operation anddisplays the resultant echo. The flexible cable 34 permits thepositioning of the module 28 at any desired location on the subject 24.

Referring also to FIG. 2, the dimensions of an exemplary array 22 arepresented. The array is seen to have a width of 13 mm (millimeters), alength of 130 mm, and includes 260 elements, each element being one ofthe transducers 40 of FIG. 1. As noted in FIG. 1, the frequency oftransmission of the sonic energy is given as 1.5 MHz (megahertz) with awavelength of 1 mm within the subject 24. Each transducer 40 has a frontface in the shape of a narrow rectangle wherein the length of each faceis 13 mm and the width thereof is 1/2 mm. Also, by way of example inreceiving sonic energy, a group of 60 elements is shown as the activeregion whereby the Fresnel patterns are formed.

Referring now to FIG. 3, the group of elements of the array 22 utilizedfor receiving sonic energy is enclosed by a bracket identified by thelegend R while another group of elements utilized for transmitting sonicenergy is enclosed by a bracket identified by the legend T. Thesubscripts 1 and 2 appended after the legends R and T indicatesubsequent positions of the group of active elements with the subscriptN indicating the final position of the receiving group at the end of ascan along the subject 24 of FIG. 1. The line 42 represents the linearstepping of the group of active elements after each scanning of a groupof sites within the subject 24 on a line parallel to the Y axis. Whilethe line 42 is shown as a straight line, it is to be understood that theactual positions of the centers of the groups of active elements aredisplaced a step at a time wherein each step may have the width of onetransducer 40 or two or more of the transducers 40.

The size of the group of elements T utilized for transmission is smallerthan the receiving group R in the simple case wherein the transmittedbeam is provided by energizing each transducer element of the group Twith signals having a common amplitude and phase; the small groupproviding a narrower beam than a larger group in the region immediatelyin front of the array 22, the region being referred to as the near fieldor Fresnel region. In the more complex case to be described hereinafterwith reference to FIG. 11, the size of the transmitting group T isgenerally equal to the size of the receiving group R since the system ofFIG. 11 provides for energizing the transducer elements with signalsmodulated in accordance with a Fresnel pattern to focus the sonicradiation at a site within the subject 24.

Referring now to FIG. 4, the first graph depicts a cosinusoidal Fresnelpattern constructed for a set of elements of the array 22 whichcomprises the active group of receiving elements. Thus, with referenceto FIG. 2, wherein sixty transducers 40 serve as the active elements,the Fresnel pattern of the first graph of FIG. 4 encompasses the sixtyelements. Similar comments apply to the second and third graphs whichrepresent a square wave approximation to the Fresnel pattern.Furthermore, with reference to the third graph of FIG. 4, it is notedthat the graph shows, by way of example, transducer signal samples thathave been multiplied by zero as well as by +1 and -1. Thus, the positivevalues indicate a multiplication by +1, while the negative valuesindicate multiplication by the factor -1. It is also noted that thethird graph shows a relatively large number of transducer elements ofwhich the signals have a common phase within the central portion of theFresnel function. Nearer the edges of the Fresnel function, the numbersof transducers involved in any one small band of the Fresnel function isrelatively small with only one transducer element being shown for thelast band. The multiplying factors utilized in the third graph producethe radiation pattern of the active region portrayed in the fourth graphwherein it is seen that there are two superposed radiation patterns. Oneof these superposed radiation patterns converges to a real focal pointin front of the array 22 of FIG. 1 while the second of the superposedradiation pattern diverges from a virtual focus which would be locatedbehind the array 22 of FIG. 1. In view of the fact that the cosinusoidalFresnel pattern is an even function of distance along the face of thearray 22 while the sinusoidal Fresnel pattern is an odd function ofdistance along the face of the array 22, the combination of the twopatterns result in the removal of the diverging pattern so that, as aresult of the signal processing of the invention, only the convergingradiation pattern is utilized in forming images of the sites within thesubject 24 of FIG. 1.

The two graphs of FIG. 5 show the radiation pattern after combination ofthe signals of the transducers of the array 22, the first graph relatingto single one of the aforementioned Fresnel patterns while the secondgraph relates to the use of both cosinusoidal and sinusoidal Fresnelpatterns. The horizontal axis of each graph represents the anglemeasured relative to a normal to the face of the array, the angles beingidentified relative to a normal in the fourth graph of FIG. 4. The firstgraph of FIG. 5 represents the prior art showing substantial noise inthe skirts 44 of the radiation pattern. The skirts 46 of the secondgraph of FIG. 5 show greatly diminished energy content therebyindicating that the signal processing of the invention involving the useof the aforementioned pair of Fresnel patterns has greatly reduced thenoise surrounding the desired signals from which the image of thesubject 24 is composed. It is also noted that, with the aforementioneduse of a Fresnel pattern in nuclear medicine, the imaging in the nuclearmedicine case is based on a non-defraction of gamma rays while, in thepresent case of sonic imaging, defraction and interference phenomena ofsonic waves produce the focussing of the radiation pattern upon a focalpoint in a manner analogous to the Fresnel focussing in optics.

Referring now to FIG. 6, the transmitter 30, the receiver 32 and theamplifier system 26 of FIG. 1 are presented in greater detail. Thetransmitter 30 is seen to comprise a modulator 48, a timer 50, anamplifier 52 and a signal splitter 54. The receiver 32 comprises adisplay 56 for portraying an image of the subject 24 of FIG. 1. Thereceiver 32 further comprises a signal generator 58 which provides online 60 a cosinusoidal carrier of the signal transmitted by the array 22as well as a sinusoidal carrier on line 61 for use in an alternativeembodiment disclosed hereinafter with reference to FIG. 11 for focussingthe sonic energy at a site within the subject 24. The generator 58 alsoprovides a pair of reference signals on lines 62 and 64 which will beutilized in a manner to be described with reference to FIG. 8 forprocessing signals received by the array 22 to provide the image of thesubject 24. The amplifier system 26 comprises a set of amplifiers 66having their respective input terminals connected by lines seen fanninginto the line 68 which is coupled to the signal splitter 54, a set oftransmit-receive circuits 70 coupled between respective output terminalsof the amplifiers 66 and the transducers 40 of the array 22, and a setof preamplifiers 72 coupled to respective ones of the circuits 70 foramplifying signals received by respective ones of the transducers 40,the output terminals of the preamplifiers 72 being coupled via lineswhich fan into line 74 for coupling via the cable 34 to the receiver 32.

The operation of the transmitter 30 and the operation of the receiver 32are synchronized by clock signals provided by the timer 50. In responseto the clock signals from the timer 50, the generator 58 applies thecosinusoidal carrier signal of line 60 to the modulator 48, and themodulator 48 applies an amplitude modulation in the form of a shortpulse to the carrier signal. By way of example, the pulse durationprovided by the modulator 48 is approximately 3 microseconds to providefour or five cycles of the carrier signal. The amplifier 52 amplifiesthe power of the pulsed signal of the modulator 48 for driving thesignal splitter 54 which will be described with reference to FIG. 9. Thetwo reference signals of the signal generator 58 on lines 62 and 64 areat double the frequency of the carrier signals, one of the referencesignals having a cosinusoidal waveform and the other reference signalhaving a sinusoidal waveform. The signal splitter 54 selects a group oftransducers 40, corresponding to the transmitting group of FIG. 3, anddistributes the pulsed carrier signal from the amplifier 52 via a set ofconductors represented by line 68 in the cable 34 to the respective onesof the amplifiers 66. The signal splitter 54, in using only the carrierof line 60, energizes equally each transducer 40 for focussing the sonicenergy at infinity. The amplifiers 66 provide sufficient power to thepulsed carrier signals for driving the transducers 40 for insonifyingthe subject 24 of FIG. 1. The circuits 70 couple the transmitted signalfrom the respective amplifiers 66 to the transducers 40 while isolatingthe signals from the preamplifiers 72. The signals received by thetransducers 40 are coupled via the circuits 70 and the preamplifiers 72to the receiver 32.

Referring now to FIG. 7, there is seen a detailed block diagram of thereceiver 32 of FIGS. 1 and 6. Line 74 of the cable 34 is seen to becoupled to the receiver 32 as well as clock signals at terminal C fromthe timer 50 as was noted hereinabove with reference to FIG. 6. Thereceiver 32 comprises a first set of multipliers 76, a second set ofmultipliers 77, summers 80 and 81, mixers 84 and 85, attenuators 88 and89 which are mechanically coupled via line 92 to a knob 94, a summer 96,a bandpass filter 98, a controller 100, and the display 56 and thesignal generator 58 which were previously seen in FIG. 6. Individuallines 102A-C, which fan out from the cable represented by the line 74,each carry the signal of one of the transducers 40 of FIG. 6, and eachis coupled to a pair of multipliers 76 and 77. For example, the line102A is coupled to multiplier #1 and multiplier #2. The line 102B iscoupled to the multiplier #3 and the multiplier #4. There are a total ofM multipliers 76-77 where M is equal to twice the number of transducers40.

The set of multipliers 76, this comprising the odd numbered multipliers,provides multiplication by the set of factors corresponding to thecosinusoidal Fresnel pattern and may be referred to hereinafter as thecosine branch. The set of multipliers 77, this being the even numberedmultipliers, provides multiplication by the set of factors correspondingto the sinusoidal Fresnel pattern and may be referred to hereinafter. asthe sine branch. Thus, the signal from each transducer is applied to onemultiplier of each set. Thereby, each transducer 40 of the active regionfor receiving radiation, as disclosed in FIG. 3, provides a contributionto the generation of the cosinusoidal Fresnel pattern and the sinusoidalFresnel pattern. The product of each of the multipliers 76-77 appears atterminal B, the input terminals of the summers 80-81 being similarlylabeled with the legend B but being further identified by the numeralscorresponding to the individual ones of the multipliers 76-77. Thus,terminal B of multiplier #1 is connected to terminal B1 of the summer80, with similar connections being applied to the other multipliers suchas the connection of terminal B of the multiplier #4 to terminal B4 ofthe summer 81. In this way, the products of each of the odd numberedmultipliers 76 are summed together by the summer 80, and the products ofeach of the even numbered multipliers 77 are summed together by thesummer 81.

Each of the multipliers 76-77 provides multiplication by a factor of +1,-1 or 0 as is shown in the block representing the multiplier #1. Each ofthe multipliers is comprised of an amplifier 104 and a switch 106 as isshown in the block representing the multiplier #2. The amplifier 104provides positive and negative polarities of the signal at its inputterminal, such as a signal on line 102A, the signals of positive andnegative polarity being coupled to two input terminals of the switch106. A third input terminal of the switch 106 is grounded. Terminal B ofthe multipliers 76-77 is selectively coupled by the switch 106 to one ofthe output terminals of the amplifier 104 whereby the product appearingat terminal B includes one of the aforementioned multiplying factors.The switch 106 in each of the multipliers 76-77 is controlled via asignal, such as a two-bit digital signal, at terminal A. The signalscoupled to the terminals A in each of the multipliers 76-77 are providedby the controller 100 which is seen to have a set of output terminalsidentified by the legend A, the identification of the output terminalsfurther including the numerals 1-M to identify the specific one of themultipliers 76-77 to which the switch control signal is being applied.The controller 100 is described briefly in FIG. 7 and in greater detailin FIG. 8, FIG. 7 showing a memory 108, an address generator 110 and arange counter 112. In accordance with the range or depth of a sitewithin the subject 24 of FIG. 1, the counter 112 counting the range ofthe site, the generator 110 addresses the memory 108 to provide the setof multiplying factors for a Fresnel pattern focussed at that site.

The sum produced by the summer 80 is multiplied in the mixer 84 by thecosinusoidal reference of the generator 58 on line 62, the mixerproviding the difference between the reference frequency and thetransmitted frequency in the output product. The mixer 85 operates inthe same fashion as does the mixer 84 to provide the product of the sumof the summer 81 and the sinusoidal reference signal. As was mentionedearlier with reference to FIG. 6, the cosinusoidal reference on line 62and the sinusoidal reference on line 64 are both at double the frequencyof the carrier from the transmitted signal. The amplitudes of theproducts of the mixers 84 and 85 are equalized by the attenuators 88 and89, the attenuators 88-89 being operated by the knob 94 for varying theattenuation of the product of the mixer 84 relative to that of the mixer85 to produce the desired equalization. Thereupon, the attenuatedsignals as provided by the attenuators 88-89 are summed together by thesummer 96 and coupled via the filter 98 to the display 56. The filter 98has a pass band sufficiently wide to pass the sum signal of the summer96 while attenuating harmonics thereof resulting from the action of themixers 84-85. A graph within the block representing the filter 98 showsexemplary cut-off frequencies at 1/2 and 3/2 of the transmittedfrequency. The signal produced by the filter 98 represents the image ofone of the sites in the subject 24 of FIG. 1 as is produced by theconverging radiation pattern of FIG. 4, the diverging radiation patternhaving been canceled out in the summer 96. The range counter 112 iscoupled to the display 56 to provide a display of images of the sites bythe filter 98 as a function of range provided by the summer 112.

Referring now to FIG. 8, the controller 100 is seen to comprise thememory 108, the address generator 110 and the range counter 112 whichwas previously seen in FIG. 7. In addition, the controller 100 comprisesa switching matrix 114, a counter 116, buffer storage registers 118-119,a switch 122 and a digital inverter 124. The switching matrix 114comprises a set of switches 126 and a set of switches 128. As notedhereinabove with reference to FIG. 7, the memory 108 stores sets offactors, these factors being presented graphically in FIG. 8 in adiagrammatic representation of the storage wherein individual rowscorrespond to depth of site as measured in the Y direction of thecoordinate system 36 in FIG. 1. Thus, by way of example, a rowcorresponds to Y=20 mm, Y=40 mm with further rows corresponding toincrements of 20 mm until the last row shown as Y=160 mm. Each rowstores the factors for both the cosinusoidal Fresnel pattern and thesinusoidal Fresnel pattern, this corresponding to the odd numberedmultipliers 76 and the even numbered multipliers 77 of FIG. 7. Anaddress for addressing individual cells of the memory 108 is provided online 130 from the generator 110. The stored data of the memory 108 isread-out on line 132 in response to the address on line 130, the storeddata on line 132 being coupled by the switch 122 alternately to theregister 118 and the register 119. As seen in FIG. 2, the number ofactive elements in the region of the array 22 used for forming theFresnel focussing is represented by the letter K. In the exampledescribed with reference to the FIGS. 2 and 4, K is assumed equal to 60.Accordingly, instead of storing pairs of factors for all 260 elements ofthe array 22 of FIG. 2, the memory 108 stores pairs of factors for eachof the 60 elements of the active region, this totaling 120 factors foreach value of Y. Accordingly, each row of the memory 108 of FIG. 8 has120 cells for storing the sixty factors of the cosinusoidal Fresnelpattern and the sixty factors of the sinusoidal Fresnel pattern. Thedesired factors are read out serially on line 132 but utilizedsimultaneously by the multipliers 76-77 of FIG. 7. The registers 118-119provide buffer storage of these factors to permit both the serialread-out on line 132 and the simultaneous control of multiplication bysignals at the terminals A1-AM.

The coupling of the factors from the line 132 to the terminals A of thecontroller 100 is accomplished as follows. The switches 128 selectalternately data stored in the registers 118-119. The switches 128 arecoupled to the same one of the registers 118-119. Thus, as seen in FIG.8, a switch 128 is seen coupling signals from the register 118 and,accordingly, the switch 122 is seen coupling signals into the register119. In this way, signals are read-out from the registers 118 inparallel via the K output lines to respective ones of the switches 128while the register 119 is being filled with new values of data stored inthe memory 108. The output signals of the switches 128, shown on lines#1, #2 and #K are applied to respective ones of the switches 126. Eachswitch 126 is in the form of a multiple selector switch or multiplierand has a set of output terminals equal in number to the number ofterminals A. Thereby, in response to a digital signal from the counter116, each switch 126 couples the signal from its input terminal to oneof the terminals A1-AM.

The operation of the switches 126 may be further explained by way ofexample. In response to clock pulses provided by the timer 50 atterminal C, the counter 116 counts successive range scans of a set ofsites within the subject 24 such as the three sites shown in FIG. 1.Upon completion of each range scan, a clock pulse is applied to theinput terminal of the counter 116, whereupon the counter advances itscount to indicate the next position along the X axis of FIG. 1 for thenext range scan along the Y axis of FIG. 1. The coupling of the switches126 to the terminals A1-AM is accomplished in a manner whereby, inresponse to a digital signal representing a count of one from thecounter 116, the first of the switches 126 couples the signal from line#1 to terminal A1, the second of the switches 126 couples the signalfrom line #2 to terminal A2, and similarly with the remaining ones ofthe K switches 126. In this way, the counter 116 in conjunction with theswitches 126 accomplishes the side-stepping of the range scans along theX axis of FIG. 1 to produce the image in the form of a swath along the Xaxis of the subject 24.

The range counter 112, in response to clock pulses provided by the timer50 by the terminal C, counts individual increments of range or depthalong the Y axis of FIG. 1. The least significant bit drives theswitches 128 and the switch 122 to accomplish the aforementionedalternation in the use of the registers 118 and 119. The logic state ofthe least significant bit changes state with each increment in range,these changes in state operating the switches 128 to accomplish theaforementioned switching between the registers 118-119. The leastsignificant bit from the range counter 112 is coupled via the inverter124 to the switch 122, the inverter 124 complementing the logic state sothat the switch 122 is directed toward the register 119 while theswitches 128 are directed to the register 118. Clocking of data throughthe registers 118-119 is accomplished by clock pulse signals from thetimer 50 which are coupled via terminal C.

Referring now to FIG. 9, the signal splitter 54 of FIG. 6 is seen tocomprise a set of switches 136 coupled to respective ones of thetransducers 40, and an address generator 138 coupled via lines 140 toindividual ones of the switches 136. The generator 138, in response tothe clock pulse signals from the timer 50, addresses individual ones ofthe switches, via the lines 140, to couple the signal from the amplifier52 to the respective ones of the transducers 40. A group of fourswitches is addressed to energize the exemplary group of fourtransducers 40, shown in FIG. 3, for transmitting a beam of sonicenergy. Groups of the switches 136 are addressed sequentiallycorresponding to the side-stepping of the group of transducers 40 ofFIG. 3 for scanning the beam along the X coordinate system 36 of FIG. 1.

MATHEMATICAL DESCRIPTION

The foregoing system can be further described mathematically as follows.The signal received by one of the transducers, s(t,x), is given by

    s(t,x)=A(x)cos(ωt+βφ.sub.x) tm (1)

where

x is distance along the array,

t is time,

φ_(x) is a phase angle dependent on the x coordinate,

ω is radian frequency and

A(x) is amplitude dependent on the x coordinate.

The expression for the signal s_(c) (t,x), passing through the cosinebranch of FIG. 7 and appearing at the output of the mixer 84, assumingthe true Fresnel pattern shown in the first graph of FIG. 4 rather thanthe square wave approximation shown in the second graph of FIG. 4, isgiven by

    s.sub.c (t,x)=K.sub.c A(x)cos(ωt+φ.sub.x)cos(βx.sup.2)cos(2ωt) (2)

After dropping the high frequency term (3ω) which would be filtered outby the electrical circuitry, the baseband component s_(cb) (t,x), isgiven by ##EQU1## where K_(c) is an amplitude scale factor and β is aconstant in the Fresnel term. The corresponding signals of the sinebranch s_(s) (t,x) and s_(sb) (t,x), appearing at the output of themixer 85 are given by ##EQU2## where K_(s) is an amplitude scale factor.

Upon adjusting the attenuators 88 and 89 to equalize the amplitudesK_(c) A(x) and K_(s) A(x) of the signals of the cosine and sinebranches, the sum of the signals of the cosine and sine branches,v_(out) (t,x), appearing at the output of the summer 96 is given by thesum of Equations (3) and (5), namely,

    v.sub.out (t,x)=KA(x)cos(ωt-φ.sub.x -βx.sup.2) (6)

The expression of Equation (6) contains only one term with βx² while anextra term in βx² appears in both Equations (3) and (5). It is theseextra terms which produce the undesired diverging beam of the prior art.When β is adjusted so that φ_(x) =-βx², the transducers are focussed ona wave with curvature about the desired focal point. The above analysisdescribes receiver operation; for a transmitter, such as that to bedescribed in FIG. 11, the mathematical analysis is similar.

This system can be made to dynamically focus, that is, the focal lengthof the transducer array can be changed, as noted hereinabove, as afunction of time to track the return echoes. This is accomplished bychanging the switches 106 of FIG. 7 in time so the quadratic term βx²matches the curvature of the returning echoes.

FOCUSSED TRANSMISSION

Referring now to FIG. 10, there is seen an alternative embodiment of thesignal splitter 54 of FIG. 9, this alternative embodiment beingidentified by the legend 54A in FIG. 10. The signal splitter 54Aprovides for improved transmission of sonic energy as compared to theoperation of the signal splitter 54, the signal splitter 54A providingfor the focussing of the sonic energy at the sites in the subject 24 ofFIG. 1 while, with the foregoing use of the signal splitter 54 in FIG.6, the sonic energy is focussed at infinity. The signal splitter 54Acomprises multipliers 77A and the controller 100 of FIGS. 7 and 8. Themultipliers 77A differ from the multiplier 77 of FIG. 7, the multiplier77A comprising an amplifier 104A and a switch 106A which are adapted forhandling relatively high signal power for the transmission of sonicenergy while the amplifier 104 and the switch 106 of the multiplier 77of FIG. 7 are adapted for use with the relatively low signal power ofsignals received by the transducers 40. The multipliers 77A of FIG. 10are controlled by the controller 100 in response to clock signals by thetimer 50 in a manner analogous to the controlling of the multipliers 77of FIG. 7 by the controller 100 as is explained hereinabove.

Since the signal splitter 54A comprises only one set of multipliers 77A,this corresponding to the sine branch of FIG. 7, the phasing of thetransducer signals follows that portrayed in the third graph of FIG. 4.Thus, the signal splitter 54A when utilized in the transmitter 30 ofFIG. 6, in lieu of the signal splitter 54, produces the square waveapproximation to the Fresnel function as described in FIG. 4. As aresult, a portion of the sonic radiation converges on the real focalpoint at one of the sites in the subject 24 of FIG. 1 while theremaining portion diverges from a virtual focus behind the array 22 ofFIGS. 1 and 4. The number of transducer elements 40 of FIG. 3 utilizedfor transmission with the signal splitter 54A, as well as that to bedescribed hereinafter with reference to the signal splitter 54B of FIGS.11 and 12, is advantageously made the same as that utilized in the groupof receiving elements designated by the letter R in FIG. 3. Thereby, theaperture size of the active elements of the array 22 is the same on bothreceiving and transmitting for an equal focussing capability.

Referring now to FIG. 11, there is seen the transducer array 22, theamplifier system 26 and the receiver 32 as disclosed previously withreference to FIG. 6. In addition, a transmitter 30A, in lieu of thetransmitter 30 of FIG. 6, is shown coupled between the receiver 32 andthe amplifier system 26. The transmitter 30A differs from thetransmitter 30 in that a pair of modulators 48 and a pair of amplifiers52A are utilized in lieu of the single modulator 48 and the singleamplifier 52 of FIG. 6. In addition, a signal splitter 54B, to befurther described with reference to FIG. 12, is shown receiving signalsfrom the amplifiers 52A, the signal on line 147 corresponding to thecosine branch, and the signal on line 148 corresponding to the sinebranch as disclosed previously with reference to FIG. 7. The amplifier52A functions in the manner of the amplifier 52 of FIG. 6, but furtherincludes a gain control terminal whereby the gain of the amplifier 52Amay be varied. A knob 150 coupled to the gain control terminals of eachof the amplifiers 52A via a mechanical connection represented by line152 decreases the gain of one of the amplifiers 52A while increasing thegain of the other of the amplifiers 52A. Thereby, rotation of the knob150 permits equalization of the amplitudes of the signals provided atthe output terminals of the two amplifiers 52A. Mathematical expressionsfor the signals of the cosine and sine branches are shown in FIG. 11adjacent the lines coupling the signals from the amplifiers 52A to thesignal splitter 54B. In the mathematical expressions, the term Arepresents the amplitude of the signal while the term M(t) representsthe modulation applied by a modulator 48. In the cosine channel, themodulation is applied to the cosinusoidal carrier signal on line 60while the modulation for the sine channel is applied to the sinusoidalcarrier signal on line 61.

Referring to both FIGS. 11 and 12, the output signal of the signalsplitter 54B on line 68 is a composite of the signals of the cosinebranch and sine branch which is produced in a manner analogous to thepreviously described operation of FIG. 7. Since the signal on line 68now contains components of both the cosinusoidal and sinusoidal Fresnelpatterns, the resultant distribution of signals across the radiatingaperture of the array 22 provides for a focussing of the transmittedsonic energy toward a real focal point in the subject 24 of FIG. 1without the production of the undesired diverging beam of sonic energyfrom the virtual focus behind the array 22 as resulted from the use ofthe signal splitter 54A of FIG. 10. The undesired diverging beam ofsonic energy is removed by the cancellation of the extra term in βx² ashas been explained with reference to Equation 6 hereinabove.

In FIG. 12 the signal splitter 54B is seen to comprise multipliers 76and 77 and the controller 100 which were previously described withreference to FIG. 7. In addition, the signal splitter 54B comprises aset of summers 154 which are coupled to the output terminals of themultipliers 76 and 77 for combining the respective signals, the linesfrom the output terminals of the summers 154 being shown fanning intothe line 68 for coupling to respective ones of the transducers 40 of thearray 22 of FIG. 11. Signals of the cosine branch are multiplied byfactors in accordance with the cosinusoidal Fresnel pattern, thesefactors being applied by the multipliers 76. Similarly, the multipliers77 provide the sinusoidal Fresnel weighting factors to the signal of thesine branch. The multipliers 76-77 are activated by the controller 100as was previously disclosed with reference to the FIGS. 7 and 8.

Referring now to FIG. 13, there is shown yet a further embodiment of thesignal splitter of FIG. 6 for providing further accuracy to theaforementioned approximation, this further embodiment being identifiedby the legend 54C. The signal splitter 54C of FIG. 13 functions in amanner analogous to that of the signal splitter 54B of FIG. 12 anddiffers therefrom by the introduction of attenuators 160-161 in lieu ofthe multipliers 76-77 of FIG. 12. While the multipliers 76-77 arerestricted to amplitudes of ±1, the attenuators 160-161 may provide anyone of a large number of amplitudes. For example, in the event that thedigital implementation of the attenuators 160 and 161 is utilized,amplitudes in increments of a quarter of the full amplitude, or evenfiner increments may be employed. Thus, with reference to the graph 164located adjacent an attenuator 161 in FIG. 13, the vectors representingthe signals of the cosine branch and the sine branch may be varied inamplitude by a small increment or a large increment to provide aresultant vector which may vary over a multiplicity of values from avalue of unity to a value of zero with phase angles φ ranging from 0° to360°. The resulting approximation is partially sketched in the secondgraph of FIG. 4 by the dashed line 166. The attenuated components areapplied by the respective attenuators 160-161 to a summer 154 to producethe desired signal amplitude and phase for each one of the transducers40 of the array 22. Thus, each transducer 40 of the radiating apertureof the array 22 receives a specific value of signal amplitude and signalphase for an accurate reproduction of the Fresnel pattern of the firstgraph of FIG. 4. In this way, a precisely focussed beam of sonic energyis produced by the system of FIG. 11 employing the signal splitter 54Cof FIG. 13 in lieu of the signal splitter 54B shown in FIG. 11. Theprecisely focussed beam of sonic energy is directed to a site within thesubject 24.

Referring also to FIGS. 14 and 15, a controller 100A of FIG. 13 isdescribed. The controller 100A provides the functions previouslydescribed with reference to the controller 100 of FIG. 8 but, inaddition, incorporates further memories 108A for steering the radiationpattern of the array 22 to provide focal points of radiant energy of thearray 22 which are situated away from a normal to the center of theradiating aperture. Each of the memories 108A provides storage functionsanalogous to that previously described with reference to the memories108 of FIG. 8 but, in addition, includes further storage space forstoring the additional amplitude values as are produced by theattenuators 160-161 of FIG. 13. As shown in FIG. 14, one of the memories108A produces a regular, or on-axis, Fresnel pattern for a focal pointlocated on the normal to the radiating aperture while a set of othermemories 108A are utilized for storing the attenuation factors for thesignals of the transducers 40 for focal points which are situated offthe normal to the radiating aperture. As seen in FIG. 15, such a focalpoint produces a skewed pattern which has the form of an off-axisFresnel pattern. With respect to the set of memories 108A producing theskewed Fresnel patterns, a separate memory is utilized for eachdisplacement of the focal point from the normal to the radiatingaperture.

In addition, to the components previously described with reference toFIG. 8, the controller 100A of FIG. 14 further comprises a switch 170coupled to each of the aforementioned memories 108A, an addressgenerator 172 for directing the switch 170 to the desired one of thememories 108A, and a knob 174 coupled to the generator 172 for eithermanually selecting a specific steering angle of the radiation pattern,or for permitting the generator 172 to automatically scan the steeringangle. The switch 170 couples the lines 130 and the lines 132,previously described in FIG. 8, to the selected one of the memories 108Afor producing a focus situated on the aforementioned normal or at apredetermined distance from the normal. In response to signals from theaddress generator 110, the selected memory 108A provides the desiredattenuation factors to the registers 118-119 via the switch 122 forstoring the attenuation values in the manner previously described forthe multiplying factors in FIG. 8. The generator 172 is responsive toclock pulses at terminal C for sequentially positioning the arms of theswitch 172 to scan the radiation pattern in angle or, alternativley asnoted above, the generator 172 addresses the switch 170 to a specificsteering angle as designated by the knob 174.

It is understood that the above-described embodiments of the inventionare illustrative only and that modifications thereof may occur to thoseskilled in the art. Accordingly, it is desired that this invention isnot to be limited to the embodiments disclosed herein but is to belimited only as defined by the appended claims.

What is claimed is:
 1. An imaging system comprising:an array ofradiating elements oriented toward a subject for communicating radiantenergy between said array and a site within said subject; first meansfor generating a first group of signals modulated in the format of acosinusoidal Fresnel pattern having an off-axis format; second means forgenerating a second group of signals modulated in the format of asinusoidal Fresnel pattern having an off-axis format; correspondingsignals of each of said groups being coupled to respective ones of saidradiating elements; and means for combining said corresponding signalsto form a Fresnel signal pattern for said array to provide a focal pointfor said radiant energy adjacent said array.
 2. A system according toclaim 1 further comprising means for selecting a portion of said arrayto which said corresponding signals are coupled and means for couplingsaid corresponding signals to said selected portion, said selectingmeans including means for successively altering selected portions ofsaid array for scanning said subject.
 3. A system according to claim 1wherein said first and said second generating means includes means foraltering said cosinusoidal and said sinusoidal Fresnel patterns to varythe distance of said focal point from said array.
 4. A signal processorfor coupling signals to an array of radiating elements for focussingradiant energy of said array at a point adjacent said array, saidprocessor comprising:means for modulating signals of respective ones ofsaid elements in accordance with formats approximating sinusoidal andcosinusoidal Fresnel patterns having an off-axis format; and meanscoupled to each modulating means for combining the modulated signals ofeach of said elements to provide a focal point of said radiant energy.5. A processor according to claim 4 further comprising means coupled tosaid modulating means for selecting the signals of specific ones of saidradiating elements to scan said focal point through a subject faced bysaid array.
 6. A processor according to claim 5 further comprising meanscoupled to said modulating means for displaying an image of saidsubject, and a transmitter including second modulating means formodulating signals for said radiating elements in the format ofsinusoidal and cosinusoidal Fresnel patterns for focussing transmittedradiant energy at a predetermined focal point in said subject.
 7. Aprocessor according to claim 4 further comprising means for displacingsaid focal point from a normal to the center of the radiating apertureof said array.